Conventionally, for a refrigerant system having a refrigerant circuit configured by the interconnection of a compressor, a condenser, an expansion valve, and an evaporator, an approach has been proposed in which a refrigerant quantity judging operation to judge the excess or deficiency of the refrigerant quantity in the refrigerant circuit is performed in order to judge the excess or deficiency of the refrigerant quantity in the refrigerant circuit (for example, see JP-A Publication No. H3-186170).